Conventionally, radar devices for detecting target objects of all the directions around a ship concerned acquire echo data of a polar coordinate system, while rotating (sweep) a radar antenna at a predetermined speed (cycle). The radar device converts the echo data of the polar coordinate system into image data of a rectangular coordinate system, and then writes the image data in an image memory. The radar device reads image data stored in the image memory at predetermined timings to display the image data. A display module of the radar device typically displays the read image data with various colors depending on its data levels. For example, a target object display device disclosed in JP 1983-53778(A) changes display color according to moving speeds of target objects, changes the display color according to kinds of target objects, such as land, a ship, and a buoy.
However, the situation of an area being detected by the radar and the situation of each target object existing in the area concerned change every moment, as well as the situation of the ship concerned which carries out the detection changes every moment. For this reason, it is difficult to correctly identify the kinds of target object echoes, such as identifying moving target object echoes and stationary target object echoes, or identifying moving target object echoes and sea surface reflection echoes.